J. Space Clim. 2020, 10,31 Ó J.-M. Malherbe et al., Published by EDP Sciences 2020 https://doi.org/10.1051/swsc/2020032 Available online at: www.swsc-journal.org Topical Issue - Space Weather Instrumentation TECHNICAL ARTICLE OPEN ACCESS

Optical instrumentation for chromospheric monitoring during 25 at Paris and Coˆ te d’Azur observatories

Jean-Marie Malherbe1,*, Thierry Corbard2, and Kevin Dalmasse3

1 LESIA, Observatoire de Paris, PSL Research University, CNRS, 92195 Meudon, France 2 Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06304 Nice, France 3 IRAP, Université de Toulouse, CNRS, CNES, UPS, 31028 Toulouse, France

Received 8 April 2020 / Accepted 21 June 2020

Abstract – We present the observing program proposed by Paris and Côte d’Azur Observatories for monitoring solar activity during the upcoming cycle 25 and providing near real time images and movies of the for space-weather research and applications. Two optical instruments are fully dedi- cated to this task and we summarize their capabilities. Short-term and fast-cadence observations of the chromosphere will be performed automatically at Calern observatory (Côte d’Azur), where dynamic events, as flare development, Moreton waves, filament instabilities and Coronal Mass Ejections onset, will be tracked. This new set of telescopes will operate in 2021 with narrow bandpass filters selecting Ha and CaII K lines. We present the instrumental design and a simulation of future images. At Meudon, the Spectroheliograph is well adapted to the long-term and low-cadence survey of chromospheric activity by recently improved and optimized spectroscopic means. Surface scans deliver daily (x, y, k) datacubes of Ha, CaII K and CaII H line profiles. We describe the nature of available data and emphasize the new calibration method of spectra.

Keywords: / chromosphere / optical observations / solar cycle 25

1 Introduction coronographs (SoHO/LASCO, ESA/NASA, since 1996; STEREO/NASA since 2006). Parker Solar Probe (NASA, Solar activity is the primary driver of space weather and launched in 2018) observes in situ the far corona and the Solar (e.g., Schrijver et al., 2012; Pomoell & Poedts, Orbiter mission (ESA) started early 2020. SDO/HMI provides 2018). It varies over a wide range of time scales from several in addition photospheric magnetograms and dopplergrams. minutes with solar flares and Coronal Mass Ejections (CMEs; We summarize below the interest of chromospheric obser- e.g., Shibata & Magara, 2011) to hundreds of years with the vations in the scope of space weather research and applications. 100-year modulation (Gleissberg cycle) of the 11-year solar Then we review the state of instrumentation dedicated to this cycle (e.g., Hathaway, 2015). Both short-term and long-term task at Paris (OP) and Côte d’Azur (OCA) observatories in full-disk observations of the Sun are thus critical for improving the context of the upcoming solar cycle 25. Section 2 presents our understanding of solar activity at all time scales, but also for new instrumentation for short-term, high-cadence monitoring monitoring and predicting it in space weather forecast applica- of solar activity. The widely open data will be used by the tions (e.g., Ueno et al., 2010; Malherbe et al., 2019). french Air Force for space weather forecast and by the scientific Ground-based observations of the chromosphere are of community to investigate unstable phenomena as flares, particular interest, because this layer is the source of solar Moreton waves or CMEs. Section 3 describes the long-term, activity. Two international networks are already operating with low-cadence Spectroheliograph while Section 4 develops the several ground-based stations at various longitudes: the Global wavelength calibration of new datacubes. Ha Network (GHN, http://ghn.njit.edu) and the GONG Ha network (http://halpha.nso.edu). Counterparts in the hot corona above are observed in EUV by Solar Dynamics Observatory 1.1 Short-term, high-cadence chromospheric (SDO/AIA, NASA, since 2010) or radiotelescopes (e.g., observations Nançay, Nobeyama). Events at large distance of the Sun (up a to 30 rad) are tracked by several wide angle, white light space High-cadence H observations are widely used in the con- text of solar flare/CME analyses and monitoring (e.g., Deng *Corresponding author: [email protected] et al., 2002; Huang et al., 2014). In the case of filament

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. J.-M. Malherbe et al.: J. Space Weather Space Clim. 2020, 10,31 eruptions, such data allow one to follow the pre-, and early-, properties of the solar cycle and its cycle-to-cycle variability. It eruptive dynamics of the filament, which provide insights into can introduce an activity asymmetry and time lag between the both the CME triggering and driving mechanisms when com- North and South solar hemispheres (similar to the one already bined with the analysis of other datasets such as photospheric observed; e.g., McIntosh et al., 2013; Svalgaard & Kamide, magnetic field data (e.g., Joshi et al., 2014; Chen et al., 2013), or trigger a grand minimum in the subsequent cycles 2018). Adopting the framework of the Torus Instability (one (similar to the ; e.g., Spoerer & Maunder, of the main models considered for CME trigger/driver; Kliem 1890; Ribes & Nesme-Ribes, 1993). & Török, 2006; Démoulin & Aulanier, 2010), then Ha imaging CaII K3 (line center) and Ha imaging can also be used to can further be used to monitor the slow rise of the filament and probe the magnetic properties of solar active regions and the predict when it will reach the critical height for the onset of the energy of solar flares in old archives, both of which are essential instability (e.g., Filippov & Zagnetko, 2008; McCauley et al., to research on extreme solar flares for the development of 2015). space weather prediction tools (e.g., Gopalswamy, 2017; Tsiftsi More generally, high-cadence Ha observations are further &DelaLuz,2018). For instance, CaII K spectroheliograms interesting because they also register the chromospheric response provide a means to estimate the magnetic flux in plages to solar flares/CMEs, as flare ribbons (e.g., Wang et al., 2003; and , thus enabling the construction of pseudo- Jiang et al., 2014)andMoretonwaves(e.g.,Narukage et al., magnetograms (e.g., Harvey & White, 1999; Pevtsov et al., 2008; Muhr et al., 2010). Flare ribbons are “surface” brighten- 2016). Using empirical laws derived from observations and ings (often observed in EUV and Ha) caused by the interaction MHD models, one can further quantify the flare-energy release of energetic particles and thermal energy (emitted at the coronal from Ha bright ribbons (e.g., Toriumi et al., 2017), and/or the reconnection region) with the lower and denser layers of the maximum energy available for a solar flare from the CaII K size solar atmosphere (e.g., Schmieder et al., 1987; Fletcher et al., and area of an active region (e.g., Aulanier et al., 2013). 2011). Moreton waves are large-scale disturbances that appear as arc-shaped, bright fronts in the center and blue wing of the Ha line (e.g., Moreton, 1960; Zhang et al., 2011) and which physical nature is still not yet fully understood (e.g., Liu et al., 2 MeteoSpace at Calern 2013; Warmuth, 2015). Both Ha flare ribbons and Moreton waves are chromospheric signatures of the coronal dynamics. The MeteoSpace instrument will operate in 2021 at Calern Their spatial and temporal evolution thus provide crucial infor- (OCA, 1270 m elevation, the sunniest station in France). It is mation on the flare/CME energetics (e.g., Gilbert et al., 2008; a fully automated system dedicated to short-term, high-cadence Veronig & Polanec, 2015), which can be used to estimate the monitoring of chromospheric activity. For that purpose, three potential impact of the flare/CME on the Earth’s magnetosphere telescopes will be available (Malherbe et al., 2019): two for in space weather forecast applications. Ha line and one for CaII K line. MeteoSpace will complete the world-wide Ha survey with 1.2 Long-term, low-cadence chromospheric enhanced features. It will join the GHN network, composed observations of many heterogeneous instruments in terms of cadence, spatial resolution, seeing, filter type and bandwidth. In terms of capa- On the other hand, long-term, low-cadence full-disk obser- bilities, MeteoSpace is close to the homogeneous GONG Ha vations of the chromosphere in CaII K and Ha lines, such as network (same filters), but it will offer improved temporal res- the centennial Kodaikanal (Hasan et al., 2010) and Meudon olution (15 s) and two Ha channels (line center and blue wing). collections (Malherbe & Dalmasse, 2019), are gold mines for MeteoSpace will also propose CaII K systematic images, with research on the solar cycle and its variability. The CaII K1v faster cadence (60 s) than previous routines (such as PSPT); imaging (blue wing) allows us to derive the sunspots number CaII K is more chromospheric than the space-borne SDO/AIA and area (e.g., Bertello et al., 2016; Chatterjee et al., 2016), 1600 and 1700 Å channels. while the Ha imaging enables us to compute the number of fil- The optical diameter of the three imagers is 100 mm with aments and their lengths (e.g., Mouradian & Soru-Escaut, 1993; 985 mm equivalent focal length. The filters bandwidth, optical Hao et al., 2015). All these quantities are good tracers of solar resolution on the Sun and observing cadence are given by activity that permit us to probe the properties of the solar cycle Table 1. Instruments are enclosed in a temperature controlled (amplitude, duration, activity level). The accumulation of such box (active heating, passive cooling) supported by an equatorial data series and products over very long periods is thus essential mount. The detectors are QSI 660i and 690i air cooled cameras to study and improve our understanding of the solar cycle vari- using shutterless interline CCD sensors from Sony (3000 ability and modulation (including the Gleissberg modulation or dynamic range, 12 bits). the occurrence of grand minima such as the Maunder minimum; MeteoSpace is the first solar automatic station in France. All e.g., Clette et al., 2015; Webb et al., 2018), both of which operations are fully robotized: weather checking, opening and directly affect space weather. closing the dome, catching the Sun, solar tracking, data The study of rare events is yet another research field to acquisition, processing and transfer to the archive, and website which long-term, full-disk CaII K1v and Ha archives contribute. update. MeteoSpace is supervised by a main computer con- Determining the frequency of rare events such as large nected to several sub-systems, for environment (weather station groups is key to both and climatology (e.g., Jiang and full-sky camera), mount steering, instrument and camera et al., 2015; Schmieder, 2018). Recent work by Nagy et al. control, data archiving and webservice. Two scintillometers (2017) shows that the emergence of a large-scale bipolar active (AiryLab company) provide an estimate of flux and seeing region into the solar atmosphere can have major effects on the conditions (100 typical at Calern) for each image. The instrument

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Table 1. Capabilities of the MeteoSpace and Spectroheliograph instruments.

Instrument CWL FWHM Cadence Simultaneous wavelengths Spatial pixel MeteoSpace Hydrogen 6562.8 Å 0.34 Å 20 s 1 0.9600 Hydrogen 6562.3 Å 0.46 Å 20 s 1 0.9600 CaIIK 3933.7 Å 1.50 Å 60 s 1 0.7800 Spectro heliograph Hydrogen 6562.8 Å 0.155 Å 2 datacubes/day >20 1.100 CaIIK 3933.7 Å 0.093 Å 2 datacubes/day >30 1.100 CaIIH 3968.5 Å 0.093 Å 2 datacubes/day >30 1.100

Fig. 1. Top: the MeteoSpace overall structure (0.5 0.5 1.7 m) enclosing three instruments; H = Ha; Ca = CaII K. Middle: details of the two Ha telescopes; E = energy rejection filter (Baader); L1 = entrance objective (Takahashi TSA102); L2/L3 = afocal magnifying system; F = DayStar Quantum PE Fabry Pérot filter; L4 = field corrector/focuser; M = focuser motor; C = cooled CCD camera (QSI 660i). Bottom: optical design of the Ha telescopes. is protected by many house-keeping procedures, including value at Calern in summer), with active heating but passive weather phenomena; in case of failure, a short messaging ser- cooling. The entrance objective (L1) is a Takahashi TSA102. vice (SMS) generates an alert to the local staff. Both incorporate an afocal magnifying system (L2/L3, 1.2) Quick look data (JPEG) will be available in real time, while for the temperature regulated Fabry-Pérot filter working in the scientific images (FITS) will be delivered a few hours later after F/30 beam (DayStar company, Quantum Pro series). The filter calibration and corrections of dark current, geometric distorsion (F) is located in the pupil plane of the chamber, which includes and rotation to present solar north up. Data will be processed L3, the filter plus a motorized field corrector/focuser (L4, M, and stored at Nice (30 Tera bytes available) but accessed Fig. 2). Flares, filament eruptions and CMEs will be observed through the BASS2000 national database dedicated to ground- with the first Ha, line centered filter. The second filter is blue based observations (http://bass2000.obspm.fr); a real-time chan- wing shifted (0.5 Å) and dedicated to the detection of fast nel is under development to disseminate high-cadence Ha Moreton waves by running differences of consecutive images, images for space weather applications. as explained by Malherbe et al. (2019). The wavelength trans- Figure 1 displays the two Ha imagers. They are enclosed mission curves of both filters are displayed in Figure 3.They in a 1.7 0.5 0.5 m3 box at 27 °C temperature (maximum were computed from Ha line profiles observed with the 14 m

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Fig. 2. The chamber of MeteoSpace Ha telescopes. L3 = chamber objective; F = DayStar Quantum PE Fabry Pérot filter; L4 = field corrector/focuser; M = focuser motor; C = cooled CCD camera (QSI 660i).

Fig. 3. The transmission curves of the line centered filter (black) and of the blue-shifted filter (blue) as a function of wavelength. The dashed line is the observed Ha line profile. spectrograph (0.02 Å spectral resolution) of the Meudon the spectrum over the filter transmission curve. The result is Solar Tower. The curves derive from filter scans by the slit, intermediate between K1v and K3 spectroheliograms (Sect. 3); after surface averaging, because the setup is a pupil plane sunspots and bright active regions are well visible, but the filter application. bandpass make the filaments vanish. The CaII K filter has a bandwidth four times larger than The simulation allows to quantify the loss of structure Hydrogen filters and provides a proxy of chromospheric contrast resulting from spectral integration over the Lorentzian magnetic fields covering facular regions. The entrance objective transmission of filters in comparison with spectroscopic of the telescope is a Takahashi FS102. The interference filter observations (Table 2). For Ha, it reveals a loss for filaments (from Barr Associates) is located in the image plane of the and faculae. On the contrary there is a gain for sunspots, motorized magnifier/focuser (1.2, not shown). because the visibility of sunspots is better in the continuum Figure 4 shows a simulation based on (x, y, k) dadacubes of and wings than in the line core. The same conclusions are the Meudon Spectroheliograph recorded on 13 Nov 2013 (near amplified for CaII K, because the filter is broader. The visibility ) during a test campaign (Sect. 3). The two Ha of prominences at the limb is not affected for Ha but is degraded images (core and blue wing centered) are obtained applying the inCaIIKforthesamereason. wavelength transfer function of the respective filters (Malherbe et al., 2019) to line profiles. Then, both images can be combined in order to provide enhanced contrast pictures of filaments, 3 Meudon Spectroheliograph plages and prominences at the limb. For that purpose, the blue wing is subtracted to the line core image after normalization Meudon Spectroheliograph operates daily since 1908 in by mean intensities measured in a large area around disk CaII K and Ha chromospheric lines. It is not a filtergraph, but center. The CaII K image is obtained also by integration of a spectrograph (Fig. 5) providing classical (k, x)spectra

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Fig. 4. Simulation of typical data that will be produced by the MeteoSpace instrument. Top (left to right): Ha core and Ha blue wing (0.5 Å). Bottom (left to right): composite Ha image showing enhanced structures (line center minus blue wing) and CaII K.

00 Table 2. Contrast loss of MeteoSpace structures compared to the 1.5 width) is located in the solar image (37.2 mm diameter). Spectroheliograph at line center. The entrance objective (O1) is mounted on a motorized transla- tor in order to scan the solar surface in 60 s typically. Solar Hydrogen Hydrogen Calcium The spectrograph has a 1.3 m focal length (O2) and uses a structure filter 1 filter 2 filter 300 groves/mm grating (17°270 blaze angle). Spectral lines form Filaments 40% 47% 57% at interference orders 3 (Ha) or 5 (CaII K and H). Orders are Faculae 40% 49% 57% selected by filters (F). The spectral resolution is about Sunspots +16% +19% +100% R = 40,000. The detector is a fast SCMOS sensor (PCO water Prominences 5% 7% 44% cooled camera, 150,00 dynamic range, 14 bits) mounted on a SkyWatcher 0.4 m apochromatic objective (O3). The through- put is 100 (k, x) spectra/s. CaII K and H are always observed (x abscissa along the slit). The second direction y, necessary to simultaneously (see Fig. 6 with solar atlas comparison). form solar images, comes from the scan of the solar surface. Long exposure observations are also done for prominences The main characteristics are reported in Table 1. (5–10 times fainter) at the limb; in order to preserve the struc- This instrument provided monochromatic images using tures of the disk, an artificial “moon” (neutral density 0.9) is glass and later film plates until 2001. The photographic device put against the solar image and is mechanically coupled to was replaced by a CCD array in 2002 (Malherbe & Dalmasse, the objective translator O1. 2019). Since mid 2017, the detector is a fast SCMOS sensor Standard corrections include dark current, curvature of which records full line profiles, so that (x, y, k) datacubes are spectral lines and rotation (solar north up). Line profile registered in 3D FITS files, as shown by the online movie datacubes(3DFITS)ofHa, CaII H and CaII K, as well as clas- (see Supplementary Material, Movie S1 for details). Images sical images (JPEG and 2D FITS derived from datacubes at line are derived from the datacubes for precise wavelength positions center, wings or continuum) are freely available to the commu- along the line profiles. nity through BASS2000 at http://bass2000.obspm.fr.Raw The spectrograph is fixed and fed by a coelostat (two datacubes (TIF) are also stored online for specificuseatftp:// flat mirrors) and a 250 mm diameter objective (O1, 4 m ftpbass2000.obspm.fr/pub/meudon/spc/tiff. Datacubes of Hb focal length). The slit (S) of the spectrograph (30 microns or line are scheduled for 2021.

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Fig. 5. The Spectroheliograph and optical scheme. S = entrance slit; SP = spectrograph; F = interference filters; M = neutral density “moon” translator for prominences; O1 = entrance objective and translator; O2 = collimator; O3 = chamber objective; A1, A2 = historical “arms” with old photographic 13 18 cm plate systems.

Fig. 6. Spectrum (k, x) between 3900 and 4000 Å displaying CaII K and H lines near disk center. Top: high resolution atlas (R =2 106, Delbouille et al., 1973). Bottom: Spectroheliograph (R = 40,000).

In order to illustrate the Spectroheliograph capabilities, we provides similar results). K1v and K3 are respectively the blue chose datasets recorded near the last solar maximum (13 Nov wing and line core. The composite image comes out from the 2013) with the new SCMOS detector (which was, at this difference between line center (K3) and the sum of the red moment, only available for tests). Figure 7 shows images and blue wings (K1r +K1v at ±1.0 Å), after normalization extracted from Ha line profiles, with many sunspots, filaments by the average values measured in a quiet region near disk and active regions. The composite image comes out from the center. As for Ha, this method reveals all solar structures in a difference between line core and the sum of the red and blue synthetic image (prominences, chromosphere at the limb, spots, wings, after normalization by the average values computed in filaments, bright plages) with better contrast. a quiet Sun region near disk center. This method makes all structures of the Sun appear together in a single frame (promi- nences, chromosphere at the limb, spots, filaments, faculae) with enhanced contrast. The Doppler velocity proxy is made of the 4 Wavelength calibration of contrast of line wings (ratio between the difference and sum Spectroheliograph datacubes of red and blue wings, after normalization by disk center values). Let us call C[i, j, k] the datacubes provided by the Spectro- Figure 8 displays images derived from CaII K line profiles heliograph FITS files with i, j, k varying from 1 to respectively (CaII H line is also available, as observed simultaneously, and NAXIS1, NAXIS2 and NAXIS3 (pixel numbers). They need to

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Fig. 7. Typical images issued from Ha datacubes produced by the Spectroheliograph. Top (left to right): continuum (1.07 Å), blue wing (0.43 Å) and line core. Bottom (left to right): limb prominences, composite image (line center minus red and blue wings) and Doppler proxy (contrast between both wings). be calibrated along the wavelength axis (k) in order to compute The minimum value of this smoothed spectra is located at header CRPIX3 and CRVAL3 keywords (line center in pixel ks(n) which is a function of n and is slightly different from unit and in Å, respectively, see Greisen & Calabretta, 2002). kc. The wavelengths corresponding to the different images of They are roughly known in advance but can be precisely deter- the cube are obtained by mined. We first locate the minimum of the integrated intensities k ¼ k ðnÞþðk k Þk of the cube images: k s min P Ci½; j; k i;j where Dk (the wavelength increment in Å) is given by the FITS Sobs½¼k k ¼ 1; :::; NAXIS3 ð1Þ NAXIS1 NAXIS2 keyword CDELT3: k ¼ CDELT3 kmin ¼ index of min ðÞSobs½k : ð2Þ 8 > : A for CaIIH ð : A before = = Þ <> 0 0929 0 0992 2017 10 15 The minimum of S at index k may not be strictly at the line ¼ 0:0932 A for CaIIK ð0:0995 A before 2017=10=15Þ ð4Þ obs min > core location kc (keyword WAVELNTH). The idea is then to fit : 0:1553 A for Ha ð0:1660 A before 2017=10=15Þ the observed line profile with a reference one Sref(k) in order to identify precisely the location of the minimum and assign to it k the value of c. The reference spectrum is taken from the visible and kmin is an unknown fractional index. We assign kc to the atlas by Delbouille et al. (1973) available online at the keyword CRVAL3 and compute CRPIX3 through: BASS2000 website (http://bass2000.obspm.fr) with a resolution k k ðnÞ dk c s of = 0.002 Å. The bandwidth of the cube is assumed CRPIX3 ¼ kmin þ : unknown and therefore the reference spectrum is smoothed over k fi a number of points n that we also have to determine. We de ne A linear scaling between the smoothed reference spectra and the S ðkÞ the smoothed reference spectrum ref to which we want to observed one may not be always appropriate far from the line compare the observed one by: core. We allow a wavelength dependence of the linear term Z kþdk and write (see top panel of Fig. 9): 1 2 Sref ðÞ¼k Sref k dk with dk ¼ n dk: ð3Þ dk kdk ^ 2 Sobs½¼k a þ bðÞkk Sref ðÞkk k ¼ 1; :::; NAXIS3 ð5Þ

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Fig. 8. Typical images issued from CaII K datacubes of the Spectroheliograph. Top (left to right): K1v (blue wing, 1.0 Å) and K3 (line core). Bottom (left to right): limb prominences (K3) and composite image (line center minus red and blue wings).

where: wk½¼ 1 : 4 2 ðkk ksðnÞÞ þ 1 bðkkÞ¼b þ cðkk kcÞþdðkk kcÞ :

The six free parameters of the model are therefore: a, b, c, d, n The six model parameters are thus obtained by minimizing a and k . They could be obtained by minimizing the sum of weighted sum of squared residuals (WSSR): min P squared residuals: ^ 2 X wk½Sref ½k Sref ðÞkk ^ 2 k SSR ¼ Sobs½kSobs½k : ð6Þ WSSRðÞ¼a; b; c; d; n; kmin P : k wk½ k

This quantity however will depend on the scaling parameters We perform a multidimensional minimization using the Nelder- and will not permit an easy comparison for the goodness of Mead simplex algorithm (Gao & Han, 2012). For the initial the fit between different data cubes (quality index). We there- parameters we take n =2Dk/dk, kmin is obtained from equation fore prefer instead to build, from the observed profile, a profile (2) and, given these two values, a, b, c, d are the least-squares that can always be compared to the reference one. From equa- solution to the linear matrix (5) which minimizes the SSR of tion (5),wecandefine: equation (6). S ½k a ForCaIIKspectralline,Figure 9 shows the observed inte- S^ ½¼k obs : ð7Þ ref grated intensities Sobs compared to the scaled smoothed refer- bðÞkk ^ ence spectrum Sobs using the initial parameters (middle panel) and after optimization (bottom panel). Such calibrations are per- Furthermore, we use a weighting function chosen in order to formed daily for CaII H, K and Ha lines; the result is available give maximum weight to the points close to the line core: at ftp://solar-ftp.oca.eu/SHG/.

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Fig. 9. Example of wavelength calibration of CaII K spectral line. The red crosses represent the observed disk-integrated intensities Sobs ^ ^ (Eq. (1)). The black lines are the scaled smoothed reference spectrum Sobs (Eq. (5)). Top panel: Sobs and Sobs are plotted as a function of the ^ intensities of the smoothed reference spectrum Sref (Eq. (3)). Middle and bottom panels: Sobs and Sobs are plotted as a function of wavelength. The coefficients a, b, c and d of the fit are shown. The vertical blue line gives the line core position at kc = 3933.663 Å. The corresponding FITS index is given by CRPIX3. The interval over which the reference spectra is smoothed is given by dk. WSSR is the resulting weighted sum of squared residuals. The top and middle panels give the fit with the initial parameters and the bottom panel gives the fit and parameters after optimization (lower WSSR).

5 Conclusions for Stokes V measurements (magnetograms) in the blue wing of NaD1 5896 Å line. New solar cycle 25 will be intensively observed at Paris and The Spectroheliograph is dedicated to long-term, low- Côte d’Azur observatories with new and fast automatic instru- cadence observations of the chromosphere and now delivers ments (MeteoSpace at Calern in 2021) or with improved capa- (x, y, k) datacubes of Ha, CaII H and K spectral lines, from which bilities existing ones (Meudon Spectroheliograph). classical images are derived. A precise wavelength calibration The MeteoSpace instrument is a new set of three automated method of datacubes, using comparison with atlas spectra, is pro- telescopes using narrow Fabry-Pérot filters and dedicated to posed. Observation of Hb line is the next extension. short-term, high-cadence (20 s) imagery of fast events such as All data are (or will be) freely available to the scientific com- flares, filament eruption, CME onset and Moreton waves. Sim- munity through a dedicated web service (BASS2000), which ulations of Ha blue wing and line core images, as well as CaII K will offer an additional real-time channel to disseminate high (magnetic proxy), are shown. In comparison with spectroscopic cadence MeteoSpace images for space weather applications. data, the Lorentzian shape of filters reduces the contrast of struc- tures (40% on filaments and faculae in Ha), but we suggest a simple method combining them to produce composite and Supplementary material enhanced images. Low cost pre-filters could also be added to limit the contrast loss to 30%. Capabilities could be extended Supplementary material is available at https://www.swsc- at the end of the space-borne SDO mission by a fourth telescope journal.org/10.1051/swsc/2020032/olm

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Cite this article as: Malherbe J-M, Corbard T & Dalmasse K 2020. Optical instrumentation for chromospheric monitoring during solar cycle 25 at Paris and Côte d’Azur observatories. J. Space Weather Space Clim. 10, 31.

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